U.S. patent application number 12/854289 was filed with the patent office on 2011-02-24 for polarization multiplexed optical transmitter and method for controlling polarization multiplexed optical signal.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Yuichi AKIYAMA, Noriaki MIZUGUCHI.
Application Number | 20110044702 12/854289 |
Document ID | / |
Family ID | 43605467 |
Filed Date | 2011-02-24 |
United States Patent
Application |
20110044702 |
Kind Code |
A1 |
MIZUGUCHI; Noriaki ; et
al. |
February 24, 2011 |
POLARIZATION MULTIPLEXED OPTICAL TRANSMITTER AND METHOD FOR
CONTROLLING POLARIZATION MULTIPLEXED OPTICAL SIGNAL
Abstract
A polarization multiplexed optical transmitter includes first
and second modulation units, combiner, phase controller, and signal
controller. The first and second modulation units generate first
and second modulated optical signals, respectively. The first and
second modulation units include first and second phase shifter to
give phase difference between optical paths of corresponding
Mach-Zehnder interferometer, respectively. The combiner generates
polarization multiplexed optical signal from the first and second
modulated optical signals. The phase controller controls the phase
difference by the first phase shifter to a target value and the
phase difference by the second phase shifter to a value shifted by
.pi. from the target value. The signal controller controls
operation state of at least one of the first and second modulation
units based on optical intensity waveform of the polarization
multiplexed optical signal.
Inventors: |
MIZUGUCHI; Noriaki;
(Kawasaki, JP) ; AKIYAMA; Yuichi; (Kawasaki,
JP) |
Correspondence
Address: |
Fujitsu Patent Center;Fujitsu Management Services of America, Inc.
2318 Mill Road, Suite 1010
Alexandria
VA
22314
US
|
Assignee: |
FUJITSU LIMITED
Kawasaki-shi
JP
|
Family ID: |
43605467 |
Appl. No.: |
12/854289 |
Filed: |
August 11, 2010 |
Current U.S.
Class: |
398/184 |
Current CPC
Class: |
H04B 10/5161 20130101;
H04J 14/06 20130101; H04B 10/541 20130101; H04B 10/50577 20130101;
H04B 10/5053 20130101; H04B 10/5561 20130101 |
Class at
Publication: |
398/184 |
International
Class: |
H04B 10/04 20060101
H04B010/04 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 21, 2009 |
JP |
2009-191650 |
Claims
1. A polarization multiplexed optical transmitter comprising: a
first modulation unit to generate a first modulated optical signal
by phase modulation and intensity modulation according to first
data, the first modulation unit including a Mach-Zehnder
interferometer and a first phase shifter to give a phase difference
between optical paths of the Mach-Zehnder interferometer; a second
modulation unit to generate a second modulated optical signal by
phase modulation and intensity modulation according to second data,
the second modulation unit including a Mach-Zehnder interferometer
and a second phase shifter to give a phase difference between
optical paths of the Mach-Zehnder interferometer; a combiner to
generate a polarization multiplexed optical signal by combining the
first and second modulated optical signals; a phase controller to
control phase differences by the first and second phase shifters;
and a signal controller to control an operation state of at least
one of the first and second modulation units, wherein the phase
controller controls the phase differences by the first and second
phase shifters to a specified target value when the first and
second data are transmitted, and controls the phase difference by
the first phase shifter to the target value and the phase
difference by the second phase shifter to a value shifted by an
amount of .pi. from the target value during an adjustment
operation; a data pattern of the first data is same as the second
data or reversed pattern of the second data during the adjustment
operation; and the signal controller controls an operation state of
at least one of the first and second modulation units based on an
optical intensity waveform of the polarization multiplexed optical
signal during the adjustment operation.
2. The polarization multiplexed optical transmitter according to
claim 1, wherein the signal controller controls an operation state
of at least one of the first and second modulation units so that an
optical intensity of the polarization multiplexed optical signal is
maintained at a constant level.
3. The polarization multiplexed optical transmitter according to
claim 1, wherein at least one of the first and second modulation
units has a delay element to delay a corresponding modulated
optical signal; and the signal controller extracts a first function
component from the optical intensity waveform of the polarization
multiplexed optical signal, and controls a delay time of the delay
element according to the first function component.
4. The polarization multiplexed optical transmitter according to
claim 3, wherein the first function represents a pair of a positive
pulse and a negative pulse; and the signal controller calculates an
adjustment amount of the delay element based on an amplitude or a
pulse width of the positive pulse or the negative pulse.
5. The polarization multiplexed optical transmitter according to
claim 1, wherein each of the first and second modulation units has
a modulator and a driver to drive the modulator; and the signal
controller extracts a second function component from the optical
intensity waveform of the polarization multiplexed optical signal,
and controls a response speed of the driver of at least one of the
first and second modulation units according to the second function
component.
6. The polarization multiplexed optical transmitter according to
claim 5, wherein the second function represents a pair of positive
pulses or a pair of negative pulses; and the signal controller
calculates an adjustment amount of the driver based on an amplitude
or a pulse width of the positive pulse or the negative pulse.
7. The polarization multiplexed optical transmitter according to
claim 1, wherein at least one of the first and second modulation
units has a power adjustment element to adjust a power of a
corresponding modulated optical signal, and the signal controller
extracts a third function component from the optical intensity
waveform of the polarization multiplexed optical signal, and
controls the power adjustment element according to the third
function component.
8. The polarization multiplexed optical transmitter according to
claim 7, wherein the third function represents a positive pulse or
a negative pulse; and the signal controller calculates an
adjustment amount of the power adjustment element based on an
amplitude of the positive pulse or the negative pulse.
9. A polarization multiplexed optical transmitter comprising: a
first modulation unit to generate a first modulated optical signal
by phase modulation and intensity modulation according to first
data; a second modulation unit to generate a second modulated
optical signal by phase modulation and intensity modulation
according to second data; a combiner to generate a polarization
multiplexed optical signal by combining the first and second
modulated optical signals; and a controller to control an operation
state of at least one of the first and second modulation units
based on the polarization multiplexed optical signal, wherein the
first modulation unit has a Mach-Zehnder interferometer; an optical
device to output the first modulated optical signal and a reversed
signal of the first modulated optical signal is provided at an
output terminal of the Mach-Zehnder interferometer; first and
second adjustment data are inserted into the identical position of
the first and second data, respectively; a data pattern of the
first adjustment data is same as the second adjustment data or
reversed pattern of the second adjustment data; and the controller
controls an operation state of at least one of the first and second
modulation units based on an optical intensity waveform of an
adjustment polarization multiplexed optical signal in which the
reversed signal of the first modulated optical signal and the
second modulated optical signal are multiplexed, in a time period
in which the first and second adjustment data are inserted.
10. A method for controlling a polarization multiplexed optical
signal in a polarization multiplexed optical transmitter comprising
a first modulation unit to generate a first modulated optical
signal by phase modulation and intensity modulation according to
first data, the first modulation unit including a Mach-Zehnder
interferometer and a first phase shifter to give a phase difference
between optical paths of the Mach-Zehnder interferometer; a second
modulation unit to generate a second modulated optical signal by
phase modulation and intensity modulation according to second data,
the second modulation unit including a Mach-Zehnder interferometer
and a second phase shifter to give a phase difference between
optical paths of the Mach-Zehnder interferometer; and a combiner to
generate a polarization multiplexed optical signal by combining the
first and second modulated optical signals, the method comprising:
controlling the phase difference by the first phase shifter to a
specified target value and the phase difference by the second phase
shifter to a value shifted by an amount of .pi. from the target
value; providing first and second control data to the first and
second modulation units, respectively, a data pattern of the first
control data being same as the second control data or reversed
pattern of the second control data; and controlling an operation
state of at least one of the first and second modulation units
based on an optical intensity waveform of the polarization
multiplexed optical signal generated when the first and second
control data are being provided to the first and second modulation
units.
11. The method according to claim 10, wherein at least one of the
first and second modulation units has a delay element to delay a
corresponding modulated optical signal; and a delay time of the
delay element is controlled so that the optical intensity waveform
is symmetry with respect to a time axis.
12. The method according to claim 10, wherein at least one of the
first and second modulation units has a power adjustment element to
adjust a power of a corresponding modulated optical signal; and the
power adjustment element is controlled so that an optical intensity
of a center part and an optical intensity of a steady level of the
optical intensity waveform match each other.
13. The method for controlling according to claim 10, wherein each
of the first and second modulation units has a modulator and a
driver to drive the modulator; and a response speed of the driver
of at least one of the first and second modulation units is
controlled so that the optical intensity waveform is maintained at
a constant level.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority of the prior Japanese Patent Application No. 2009-191650,
filed on Aug. 21, 2009, the entire contents of which are
incorporated herein by reference.
FIELD
[0002] The present invention relates a polarization multiplexed
optical transmitter and a method for controlling polarization
multiplexed optical signal. The invention may be applied to, for
example, an optical transmitter used in a polarization multiplexed
transmission system.
BACKGROUND
[0003] The needs for super high speed transmission systems with a
speed of more than 40 Gbit/s (for example, 100 Gbit/s) have been
increasing rapidly. For this reason, development is under way for
practical realization of an optical transmission system that adopts
a multilevel modulation scheme applied to a radio system (for
example, the QPSK modulation using four-level phase modulation).
However, as the transmission-signal speed becomes higher, solving
problems related to the feasibility of the electric signal circuit
and problems related to the degradation of the optical transmission
signal (transmission-signal spectrum degradation due to an optical
filter and signal degradation due to chromatic dispersion and
accumulation of optical noises) becomes more difficult.
[0004] As one of techniques for solving these problems, optical
polarization multiplexing has attracted attention. A polarization
multiplexed optical signal is generated by, for example, a
polarization multiplexed optical transmitter illustrated in FIG.
1A. The polarization multiplexed optical transmitter has a light
source (LD), a pair of modulators, and a polarization beam combiner
(PBC). Continuous wave light output from the light source is spilt
and guided to the pair of modulators. The pair of modulators
modulate the continuous wave light respectively with corresponding
data signal, and generate a pair of modulated optical signals. The
polarization beam combiner generates a polarization multiplexed
optical signal illustrated in FIG. 1B by combining the pair of
modulated optical signals. In other words, in the polarization
multiplexing, two data streams are transmitted using two polarized
waves (H polarization and V polarization) that have the same
wavelength and are orthogonal to each other.
[0005] Accordingly, in the polarization multiplexing, the data
speed becomes half, realizing the improvement of the
characteristics of the electric-signal generation circuit and
reduces the cost, size and power consumption of the circuit. In
addition, the characteristics of the optical transmission system as
a whole is improved, as influences from quality-degradation factors
such as dispersion on the optical transmission path are reduced. As
related arts, configurations described in Japanese Laid-open Patent
Publication No. 2008-172714 and Japanese Laid-open Patent
Publication No. 2009-63835 have been proposed.
[0006] However, in a polarization multiplexed optical transmitter
that generates a polarization multiplexed optical signal, a
modulator is provided for each polarization as illustrated in FIG.
1A. For this reason, the balance of characteristics between the
polarizations in the polarization multiplexed output optical signal
may deteriorate, due to factors such as manufacturing variability
of characteristics between the modulators (for example, loss of the
LN modulator) and characteristics of the optical splitter and/or
the optical combiner. When unbalance occurs in the characteristics
between the polarizations, the characteristics of the transmission
signal deteriorates.
SUMMARY
[0007] According to an aspect of an invention, a polarization
multiplexed optical transmitter includes: a first modulation unit
to generate a first modulated optical signal by phase modulation
and intensity modulation according to first data, the first
modulation unit including a Mach-Zehnder interferometer and a first
phase shifter to give a phase difference between optical paths of
the Mach-Zehnder interferometer; a second modulation unit to
generate a second modulated optical signal by phase modulation and
intensity modulation according to second data, the second
modulation unit including a Mach-Zehnder interferometer and a
second phase shifter to give a phase difference between optical
paths of the Mach-Zehnder interferometer; a combiner to generate a
polarization multiplexed optical signal by combining the first and
second modulated optical signals; a phase controller to control
phase differences by the first and second phase shifters; and a
signal controller to control an operation state of at least one of
the first and second modulation units. The phase controller
controls the phase differences by the first and second phase
shifters to a specified target value when the first and second data
are transmitted, and controls the phase difference by the first
phase shifter to the target value and the phase difference by the
second phase shifter to a value shifted by an amount of .pi. from
the target value during an adjustment operation. A data pattern of
the first data is same as the second data or reversed pattern of
the second data during the adjustment operation. The signal
controller controls an operation state of at least one of the first
and second modulation units based on an optical intensity waveform
of the polarization multiplexed optical signal during the
adjustment operation.
[0008] According to another aspect of an invention, a polarization
multiplexed optical transmitter includes: a first modulation unit
to generate a first modulated optical signal by phase modulation
and intensity modulation according to first data; a second
modulation unit to generate a second modulated optical signal by
phase modulation and intensity modulation according to second data;
a combiner to generate a polarization multiplexed optical signal by
combining the first and second modulated optical signals; and a
controller to control an operation state of at least one of the
first and second modulation units based on the polarization
multiplexed optical signal. The first modulation unit has a
Mach-Zehnder interferometer. An optical device to output the first
modulated optical signal and a reversed signal of the first
modulated optical signal is provided at an output terminal of the
Mach-Zehnder interferometer. First and second adjustment data are
inserted into the identical position of the first and second data,
respectively. A data pattern of the first adjustment data is same
as the second adjustment data or reversed pattern of the second
adjustment data. The controller controls an operation state of at
least one of the first and second modulation units based on an
optical intensity waveform of an adjustment polarization
multiplexed optical signal in which the reversed signal of the
first modulated optical signal and the second modulated optical
signal are multiplexed, in a time period in which the first and
second adjustment data are inserted.
[0009] According to another aspect of an invention, a method for
controlling a polarization multiplexed optical signal in a
polarization multiplexed optical transmitter comprising a first
modulation unit to generate a first modulated optical signal by
phase modulation and intensity modulation according to first data,
the first modulation unit including a Mach-Zehnder interferometer
and a first phase shifter to give a phase difference between
optical paths of the Mach-Zehnder interferometer; a second
modulation unit to generate a second modulated optical signal by
phase modulation and intensity modulation according to second data,
the second modulation unit including a Mach-Zehnder interferometer
and a second phase shifter to give a phase difference between
optical paths of the Mach-Zehnder interferometer; and a combiner to
generate a polarization multiplexed optical signal by combining the
first and second modulated optical signals. The method includes:
controlling the phase difference by the first phase shifter to a
specified target value and the phase difference by the second phase
shifter to a value shifted by an amount of .pi. from the target
value; providing first and second control data to the first and
second modulation units, respectively, a data pattern of the first
control data being same as the second control data or reversed
pattern of the second control data; and controlling an operation
state of at least one of the first and second modulation units
based on an optical intensity waveform of the polarization
multiplexed optical signal generated when the first and second
control data are being provided to the first and second modulation
units.
[0010] The object and advantages of the invention will be realized
and attained by means of the elements and combinations particularly
pointed out in the claims.
[0011] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are not restrictive of the invention, as
claimed.
BRIEF DESCRIPTION OF DRAWINGS
[0012] FIG. 1A and FIG.1B are diagrams describing polarization
multiplexing.
[0013] FIG. 2 is a diagram illustrating the configuration of a
polarization multiplexed optical transmitter according to an
embodiment.
[0014] FIG. 3A and FIG. 3B are diagrams illustrating BPSK
modulator.
[0015] FIG. 4A and FIG. 4B are diagrams describing the operation of
the modulator illustrated in FIG. 3A and FIG. 3B.
[0016] FIG. 5 is a diagram illustrating the operation to generate
an H polarization signal.
[0017] FIG. 6 is a diagram illustrating the operation to generate a
V polarization signal.
[0018] FIG. 7 is a diagram illustrating the polarization component
of the polarization multiplexed optical signal generated at the
time of adjustment.
[0019] FIG. 8 is a diagram illustrating the polarization component
of the polarization multiplexed optical signal at the time of data
transmission.
[0020] FIG. 9 is a diagram illustrating the power of the
polarization multiplexed optical signal and each polarization
component.
[0021] FIG. 10A-FIG. 10C are diagrams illustrating the relationship
between the timing error between polarizations and the optical
intensity of the polarization multiplexed optical signal.
[0022] FIG. 11A-FIG. 11C are diagrams illustrating the relationship
between the Tr/Tf time difference between polarizations and the
optical intensity of the polarization multiplexed optical
signal.
[0023] FIG. 12A-FIG. 12C are diagrams illustrating the relationship
between the optical power difference between polarizations and the
optical intensity of the polarization multiplexed optical
signal.
[0024] FIG. 13A, FIG. 13B, FIG. 14A and FIG. 14B are diagrams
illustrating a procedure to control the balance between
polarizations.
[0025] FIG. 15A, FIG. 15B, FIG. 16A and FIG. 16B are other
embodiments of the BPSK modulator.
[0026] FIG. 17A and FIG. 17B are diagrams illustrating a QPSK
modulator.
[0027] FIG. 18A and FIG. 18B are diagrams describing the operation
of the modulator illustrated in FIG. 17A and FIG. 17B.
[0028] FIG. 19 is a diagram illustrating the polarization component
of the polarization multiplexed optical signal in which QPSK
modulated signals are multiplexed.
[0029] FIG. 20 is a diagram illustrating the power of the
polarization multiplexed optical signal in which QPSK modulated
signals are multiplexed and each polarization component.
[0030] FIG. 21A-FIG. 21C are diagrams illustrating the relationship
between the timing error between polarizations and the optical
intensity of the polarization multiplexed optical signal in
QPSK.
[0031] FIG. 22A-FIG. 22C are diagrams illustrating the relationship
between the Tr/Tf time difference between polarizations and the
optical intensity of the polarization multiplexed optical signal in
QPSK.
[0032] FIG. 23A-FIG. 23C are diagrams illustrating the relationship
between the optical power difference between polarizations and the
optical intensity of the polarization multiplexed optical signal in
QPSK.
[0033] FIG. 24 is a diagram illustrating the configuration of a
polarization multiplexed optical transmitter having two light
sources.
[0034] FIG. 25 is a flowchart illustrating a method for adjusting
the polarization multiplex optical transmitter according to the
embodiment.
[0035] FIG. 26 is a flowchart illustrating a method for adjusting
the balance between polarizations.
[0036] FIG. 27 is a diagram illustrating the configuration of a
polarization multiplexed optical transmitter according to another
embodiment.
[0037] FIG. 28A and FIG. 28B illustrate BPSK modulator used in the
configuration illustrated in FIG. 27.
[0038] FIG. 29 illustrates data used in the configuration
illustrated in FIG. 27.
[0039] FIG. 30 illustrates a QPSK modulator used in the
configuration illustrated in FIG. 27.
DESCRIPTION OF EMBODIMENTS
[0040] FIG. 2 illustrates the configuration of a polarization
multiplexed optical transmitter according to an embodiment. The
polarization multiplexed optical transmitter according to the
embodiment transmits a polarization multiplexed optical signal
obtained by combining first and second modulated optical signals.
The polarization multiplexed optical signal carries data to a
receiving station using a pair of polarized waves (H polarization
and V polarization) that are orthogonal to each other. Here, if the
characteristics of the H polarization signal and the V polarization
signal are different from each other, the characteristics of the
polarization multiplexed optical signal deteriorate. Therefore, in
the polarization multiplexed optical transmitter according to the
embodiment, control to make the characteristics of the H
polarization signal and the H polarization signal the same as each
other or approximately the same as each other is performed. The
polarization multiplexed optical transmitter according to the
embodiment has functions to control the following three
characteristics. [0041] (1) Timing error (difference in the delay)
between the H polarization signal and the V polarization signal
[0042] (2) Difference in the rising/falling times (difference in
Tr/Tf) of the optical intensity waveforms of the H polarization
signal and the V polarization signal [0043] (3) Optical power
difference between the H polarization and the Y polarization
[0044] A light source (LD) 1 generates an optical signal of a
certain frequency. The light source 1 is, for example, a laser
diode. The optical signal generated by the light source is, for
example, a continuous wave (CW). An optical splitter 2 splits the
continuous wave light generated by the light source 1 and guides
the light to first and second modulation units. The powers of the
pair of continuous wave lights output from the optical splitter 2
are the same as each other. The optical splitter 2 is, in the
example illustrated in FIG. 2, a polarization beam splitter (PBS).
However, the optical splitter 2 does not need to be a polarization
beam splitter.
[0045] The first modulation unit has a modulator 11, a driver 13, a
delay element 14 and an optical attenuator 15. Similarly, the
second modulation unit has a modulator 21, a driver 23, a delay
element 24 and an optical attenuator 25. The pair of the continuous
wave lights output from the optical splitter 2 are given to the
modulators 11 and 21. The first and second modulation units
modulate the input CW lights in the same modulation scheme as each
other.
[0046] The modulators 11 and 21 are, in this example, modulators in
which the power of the output light periodically changes according
to the drive voltage (for example, LN modulator including a
Mach-Zehnder interferometer). Here, the modulator 11 has a phase
shifter 12 that gives a phase difference between a pair of optical
paths of the Mach-Zehnder interferometer. The modulator 11
generates a modulated optical signal H by modulating the input CW
light according to data 1. Similarly, the modulator 21 has a phase
shifter 22 that gives a phase difference between a pair of optical
paths of the Mach-Zehnder interferometer. The modulator 21
generates a modulated optical signal V by modulating the input CW
light according to data 2. The modulated optical signal H and the
modulated optical signal V are optical signals carried by the H
polarization and the V polarization, respectively.
[0047] The driver 13 generates a drive voltage signal representing
the data 1 and gives the signal to the modulator 11. Similarly, the
driver 23 generates a drive voltage signal representing the data 2
and gives the signal to the modulator 21. Meanwhile, the modulators
11 and 21 respectively have a bias control circuit that is not
illustrated in the drawing, for controlling the operating point
(that is, the DC bias) of the LN modulator. The bias control
circuit is, for example, an ABC (Auto Bias Control) circuit. For
example, the ABC circuit applies a low-frequency voltage signal to
the corresponding LN modulator, and adjusts the operating point
(that is, the DC bias voltage) of the corresponding LN modulator
based on the low-frequency component contained in the output light
of the modulators 11 and 21.
[0048] Meanwhile, while the LN modulator is described as an example
of the modulators 11 and 21 herein, the invention is not limited to
this configuration. The modulators 11 and 21 are not limited to the
LN modulator, and may be a modulator using an electro-optic
material, for example, a modulator including semiconductor material
such as InP.
[0049] Delay elements 14 and 24 are provided between the data
generators 31 and 32, and the drivers 13 and 23, respectively. The
delay element 14 delays the data 1 generated by the data generator
31. In the same manner, the delay element 24 delays the data 2
generated by the data generator 32. The delay time of the delay
elements 14 and 24 are controlled by a controller 43.
[0050] The optical attenuators 15 and 25 adjust the modulated
optical signals H and V, respectively. The optical attenuators 15
and 25 may be provided on the input side of the modulators 11 and
21, or may be provided within the modulators 11 and 21, or may be
provided on the output side of the modulators 11 and 21,
respectively.
[0051] A polarization beam combiner (PBC) 3 generates a
polarization multiplexed optical signal by combining the modulated
optical signals H and V. Here, in the polarization multiplexing, H
polarization and V polarization that are orthogonal to each other
are used, as illustrated in FIG. 1B. That is, the modulated optical
signal H (data 1) is carried using the H polarization, and the
modulated optical signal V (data 2) is carried using the V
polarization. While there is no particular limitation, for example,
the polarization multiplexed optical transmitter according to the
embodiment transmits data by DP-NRZ-MPSK (Dual Polarization NRZ
M-Phase Shift Keying) or by NRZ-MPSK, where MPSK is 2.sup.n phase
shift keying (n is an integer).
[0052] The data generators 31 and 32 generate data 1 and 2,
respectively. When the polarization multiplexed optical transmitter
transmits data to a receiving station, the data generators 31 and
32 generate transmission data 1 and 2 as the data 1 and 2,
respectively. At the time of adjusting, the data generators 31 and
32 generate control data 1 and 2, as the data 1 and 2,
respectively.
[0053] When the polarization multiplexed optical transmitter
configured as described above transmits data to a receiving
station, the phase shifters 12 and 22 are respectively controlled
to a specified target value in advance according to the modulation
scheme. Meanwhile, the data generators 31 and 32 generate the
transmission data 1 and 2 as described above. The modulator 11
generates the modulated optical signal H corresponding to the
transmission data 1 and the modulator 21 generates the modulated
optical signal V corresponding to the transmission data 2. Then,
the polarization multiplexed optical signal in which the modulated
optical signals H and V are multiplexed is output.
[0054] The polarization multiplexed optical transmitter according
to the embodiment has the following control system for optimizing
the balance of a pair of polarization signals (H polarization
signal and V polarization signal) contained in the polarization
multiplexed optical signal.
[0055] A power splitter 41 splits a portion of the polarization
multiplexed optical signal output from the polarization beam
combiner 3. The portion of the polarization multiplexed optical
signal split by the power splitter 41 contains the H polarization
signal and the V polarization signal. Meanwhile, the major portion
of the polarization multiplexed optical signal output from the
polarization beam combiner 3 is guided to an optical fiber
transmission path, for example.
[0056] An optical intensity monitor 42 has a photo detector (PD),
and converts the polarization multiplexed optical signal split by
the optical splitter 41 into an electric signal. That is, an
electric signal representing the optical intensity of the
polarization multiplexed optical signal is generated. The photo
detector is, for example, a photo diode. The optical intensity
monitor 42 preferably converts the polarization multiplexed optical
signal by a photo detector with which the polarization dependent
loss (PDL) is small enough to be regarded as negligible.
[0057] The controller 43 controls the operation and state of the
polarization multiplexed optical transmitter. In addition, the
controller 43 provides a data transmission mode and an adjustment
mode. In the data transmission mode, the polarization multiplexed
optical transmitter transmits data to a receiving station. In the
adjustment mode, devices in the polarization multiplexed optical
transmitter are adjusted for adjusting the balance between the
polarizations in the polarization multiplexed optical signal.
[0058] While there is no particular limitation, for example, the
controller 43 is realized by executing a software program using a
processor. In this case, the electric signal representing the
polarization multiplexed optical signal is converted into digital
data and then input to controller 43. Alternatively, the controller
43 may be equipped with an A/D converter.
[0059] The controller 43 has a phase controller 44, a data
controller 45, and a signal controller 46. The phase controller 44
controls the phase difference between the optical paths of the
Mach-Zehnder interferometer provided in the modulators 11 and 21,
using an optical phase control signal. That is, the phase
controller 44 controls the phase difference between the optical
paths of the Mach-Zehnder interferometer in the phase modulators 11
and 21 to a target value corresponding to the modulation scheme, by
controlling the phase shifters 12 and 22 during the data
transmission mode. Meanwhile, during the adjustment mode, the phase
controller 44 controls the phase difference between the optical
paths of the Mach-Zehnder interferometer of the modulator 11 to the
target value, and controls the phase difference between the optical
paths of the Mach-Zehnder interferometer of the modulator 21 to a
value shifted from the target value by .pi., where .pi. includes
.pi.+2n.pi. (n is an integer other than zero). Note that the phase
difference between the optical paths of the Mach-Zehnder
interferometer of each of the modulators 11 and 21 is controlled
by, for example, DC bias voltage. In this case, the correspondence
relationship between the DC bias voltage and the phase difference
may be calculated and obtained in advance. Then the DC bias for the
modulator 21 maybe shifted by the amount of voltage corresponding
to the phase difference .pi..
[0060] The data controller 45 controls the data 1 and 2 generated
by the data generators 31 and 32 using data switching instructions.
During the data transmission mode, an instruction for outputting
transmission data is given to the data generators 31 and 32.
Meanwhile, during the adjustment mode, an instruction for
outputting control data is given to the data generators 31 and 32.
The control data are generated so as to satisfy "data 1=data 2" or
"data 1=reverse pattern of data 2". In addition, data pattern of
the control data is, for example, pseudo-random.
[0061] The signal controller 46 controls at least one of first and
second modulation units according to the optical intensity of the
polarization multiplexed optical signal detected by the optical
intensity monitor 42 to improve or optimize the balance of a pair
of polarization signals contained in the polarization multiplexed
optical signal. The following three controls are performed in this
embodiment. [0062] (1) to reduce the timing error (delay time
difference) between polarizations [0063] (2) to reduce the
difference in the rising/falling times (Tr/Tf difference) of the
optical intensity waveform between polarizations [0064] (3) to
reduce the optical power difference between polarizations
[0065] The timing error between polarizations is controlled by
adjusting the delay amount of the delay elements 14 and 24 using a
delay adjustment signal. For example, when the V polarization
signal is delayed behind the H polarization signal, a delay
adjustment signal to increase the delay of the delay element 14 or
to reduce the delay of the delay element 24 is generated.
Meanwhile, the polarization multiplexed optical transmitter
according to the embodiment does not need to be equipped with both
of the delay elements 14 and 24, and may be configured to have
either one of the delay elements 14 and 24. In addition, the timing
error between the polarizations may be controlled by adjusting
other delay elements.
[0066] The difference in Tr/Tf between the polarizations is
adjusted by adjusting the drive current of the drivers 13 and 23
using a drive adjustment signal. In this example, it is assumed
that the drivers 13 and 23 respectively have an amplifier that
amplifies an input signal, and the drive current of the amplifier
is controlled according to the drive adjustment signal. In this
case, when the drive current of the amplifier increases, the
response speed of the drivers 13 and 23 increases, and the time
taken for data signals to transit between "0" and "1" is shortened.
As a result, the Tr/Tf of the modulated optical signal generated by
the modulators 11 and 21 is shortened. For example, when the Tr/Tf
of the H polarization signal is longer than the Tr/Tf of the V
polarization signal, the drive adjustment signal to increase the
drive current of the driver 13 or to reduce the drive current of
the driver 23 is generated. Meanwhile, the polarization multiplexed
optical transmitter according to the embodiment maybe configured to
control only one of the drivers 13 and 23. In addition, the
difference in Tr/Tf between the polarizations may be controlled by
adjusting other elements.
[0067] The optical power difference between the polarizations is
controlled by adjusting the attenuation amount of the optical
attenuators 15 and 25 using an optical power adjustment signal. For
example, when the optical power of the H polarization is higher
than that of the V polarization signal, the optical power
adjustment signal to increase the attenuation amount of the optical
attenuator 15 or to reduce the attenuation amount of the optical
attenuator 25 is generated. The polarization multiplexed optical
transmitter according to the embodiment does not need to be
equipped with both of the optical attenuators 15 and 25, and may be
configured to have either one of the optical attenuators 15 and 25.
In addition, the optical power difference between the polarizations
may be controlled by adjusting other elements.
[0068] As described above, according to the polarization
multiplexed optical transmitter according to the embodiment, in the
adjustment mode, feedback control to reduce (preferable, to
minimize) the timing error, the Tr/Tf difference, and the optical
power difference between the polarizations is performed. By this
feedback control, an operation status to generate a polarization
multiplexed optical signal with good quality is realized.
Therefore, by transmitting data in this operation status, the
quality of the polarization multiplexed optical signal is improved.
Note that the timing error, the Tr/Tf difference and the optical
power balance may be generated independently from each other, and
may be adjusted independently from each other.
[0069] Next, the modulation scheme of the modulators 11 and 21
provided in the polarization multiplexed optical transmitter
according to the embodiment is explained. In this example, while
there is no particular limitation, for example, the modulators 11
and 21 perform phase modulation and intensity modulation according
to input data. For the phase modulation, BPSK and QPSK are
explained below. In the intensity modulation, the optical intensity
changes in the time period in which data changes.
[0070] FIG. 3A and FIG. 3B illustrate an example of the
configuration of the modulators 11 and 21. The modulation scheme of
each of the modulators is BPSK. Each of the modulators respectively
includes a Mach-Zehnder interferometer. The Mach-Zehnder
interferometer has a pair of optical paths P1 and P2, and an input
CW light is split equally and guided to the optical paths P1 and
P2. The modulator illustrated in FIG. 3A and FIG. 3B is a single
drive configuration.
[0071] The phase difference between the optical paths P1 and P2 is
controlled to a specified target value using a phase shifter. In
the example illustrated in FIG. 3A, the phase difference between
the optical paths P1 and P2 is controlled to zero. The phase
difference between the optical paths P1 and P2 is controlled by the
DC voltage applied to the optical waveguide forming the optical
paths P1 and P2. In addition, the phase shifter to provide the
phase difference corresponds to the phase shifters 12 and 22 in the
configuration illustrated in FIG. 2. By controlling the operating
point of the modulator, for example, the phase of the output light
is modulated to ".pi." when the input data is "1", and the phase of
the output light is modulated to "0" when the input data is "0".
That is, a modulated optical signal carrying the input data is
generated.
[0072] At the time of adjusting the polarization multiplexed
optical transmitter, in one of the modulators 11 and 21, the phase
difference between the optical paths P1 and P2 is controlled to the
value shifted from the target value by .pi., as illustrated in FIG.
3B. In this example, the phase difference .pi. is added in the
modulator 21. That is, at the time of adjusting the polarization
multiplexed optical transmitter, the modulator 11 is controlled to
the state illustrated in FIG. 3A, and the modulator 21 is
controlled to the state illustrated in FIG. 3B.
[0073] FIG. 4A and FIG. 4B illustrate the operation of the
modulator illustrated in FIG. 3A and FIG. 3B. FIG. 4A and FIG. 4B
represent the electric field vector of the modulated optical
signals generated by the modulator controlled to be the state
illustrated in FIG. 3A and FIG. 3B, respectively. In the following
description, it is assumed that the modulated optical signal
generated by the modulator illustrated in FIG. 3A is used as the H
polarization signal, and the modulated optical signal generated by
the modulator illustrated in FIG. 3B is used as the V polarization
signal.
[0074] In the phase status illustrated in FIG. 3A, when the input
data is "0", the optical signal generated on the optical path P1 is
represented by a signal point A, and the optical signal generated
on the optical path P2 is also represented by the signal point A.
When the input data is "1", the optical signal generated on the
optical path P1 is represented by the signal point B, and the
optical signal generated on the optical path P2 is also represented
by the signal point B.
[0075] The electric field vector of the optical signal output from
the modulator illustrated in FIG. 3A (H polarization signal) is
obtained by compositing the electric field vector of the optical
signal generated on the optical path P1 and the electric field
vector of the optical signal generated on the optical path P2.
Thus, the electric field vector of the H polarization signal is
represented by a signal point C when the input data is "0", and
represented by a signal point D when the input data is "1". Here,
the power of the optical signal is represented by the square of the
distance from the origin to the corresponding signal point.
[0076] In the modulator configured as described above, when the
input data transits from "0" to "1", the electric field vector of
the optical signal generated on the optical path P1 moves from the
signal point A to the signal point B through a route a. The
electric field vector of the optical signal generated on the
optical path P2 also moves from the signal point A to the signal
point B through a route b. Here, the electric field vector of the H
polarization signal is obtained by compositing the two electric
field vectors. Thus, the electric field vector of the H
polarization signal moves from the signal point C to the signal
point D through the origin, when the two electric field vectors
moves from the signal point A to the signal point B. Therefore,
when the input data transit from "0" to "1", the power of the H
polarization signal once becomes zero. Similarly, when the input
data transit from "1" to "0", the electric field vector of the H
polarization signal moves from the signal point D to the signal
point C through the origin. That is, when the input data transits
from "1" to "0", the power of the H polarization also once becomes
zero. Thus, the H polarization signal generated at the time of
adjusting the polarization multiplexed optical transmitter is an
intensity modulated optical signal having an optical power of a
local minimum value (ideally, zero) in the time period in which the
input data changes(transit from "0" to "1", or transit from "1" to
"0").
[0077] In the modulator illustrated in FIG. 3B, the phase
difference between the optical paths P1 and P2 is shifted from the
target value by .pi.. For this reason, in this modulator, as
illustrated in FIG. 4B, when the input data is "0", the optical
signal generated on the optical path P1 is represented by a signal
point F, and the optical signal generated on the optical path P2 is
represented by a signal point E. When the input data is "1", the
optical signal generated on the optical path P1 is represented by
the signal point E, and the optical signal generated on the optical
path P2 is represented by the signal point F.
[0078] The electric field vector of the optical signal output from
the modulator illustrated in FIG. 3B (V polarization signal) is
represented by a signal point G (that is, the origin) in both cases
when the input data is "0" and when the input data is "1".
Therefore, the power of the V polarization signal is zero during in
the adjustment mode.
[0079] However, when the input data transit from "0" to "1", the
electric field vector of the optical signal generated on the
optical path P1 moves from the signal point F to the signal point E
through a route c. The electric field vector of the optical signal
generated on the optical path P2 moves from the signal point E to
the signal point F through the route c. Therefore, when the input
data transit from "0" to "1", the electric field vector of the V
polarization signal moves from the signal point G to a signal point
H and then returns to the signal point G. The same applies to the
case when the input data transits from "1" to "0". That is, V
polarization signal is an intensity modulated optical signal having
certain optical power only in the time period in which the input
data changes.
[0080] FIG. 5 illustrates the generation of the modulated optical
signal H used as the H polarization signal. The sine curve
illustrated in FIG. 5 represents a characteristic (drive
voltage-output optical power characteristic) of the modulator
illustrated in FIG. 3A. In this example, a drive voltage signal for
first-sixth symbol of data string is applied to the modulator.
[0081] In FIG. 5, the data of the first symbol is "0". In this
case, "V.sub.0"is applied to the modulator as the drive voltage,
and the output optical power is "1 (normalized value)". Next, the
data of the second symbol is "1". In this case, "V.sub.0+2V.pi." is
applied to the modulator, and the output optical power is "1".
Here, V.pi. represents the drive voltage for changing the optical
phase by the amount of .pi. in the Mach-Zehnder interferometer.
Thus, the power of the modulated optical signal generated by the
modulator illustrated in FIG. 3A (that is, the power of the H
polarization signal) is "1" regardless of the input data.
[0082] However, when the input data changes, the power of the H
polarization signal also changes. For example, at the time of the
transition from the first symbol to the second symbol, the drive
voltage changes from "V.sub.0" to "V.sub.0+2V.pi." continuously.
During this transition of data, the output optical power of the
modulator once decreases from "1" to "0" and then returns to "1".
That is, the H polarization signal has a local minimum value
(ideally, zero) of the optical power in the time period in which
the input data changes.
[0083] FIG. 6 illustrates the generation of the modulated optical
signal V used as the V polarization signal. In the modulator that
generates the modulated optical signal V, as illustrated in FIG.
3B, the phase difference between the optical paths of the
Mach-Zehnder interferometer is shifted by .pi.. The phase
difference .pi. is realized by applying the DC voltage V.pi. to the
modulator. Therefore, when the input data is "0", "V.sub.0+V.pi."
is applied to the modulator, and the output optical power is "0".
When the input data is "1", "V.sub.0+3V.pi." is applied to the
modulator, and the output optical power is "0". Thus, the power of
the modulated optical signal (that is, the power of the V
polarization signal) is "0" regardless of the input data.
[0084] However, similar to the H polarization signal, when the
input data changes, the power of the V polarization signal also
changes. That is, for example, at the time of the transition from
the first symbol to the second symbol, the drive voltage changes
from "V.sub.0" to "V.sub.0+3V.pi." continuously. During this
transition of data, the output power of the modulator increases
from "0" to "1" and then returns to "0". That is, the V
polarization signal has the peak of the optical power in the time
period in which the input data changes.
[0085] In the examples illustrated in FIG. 5 and FIG. 6, the H
polarization signal and V polarization signal are generated by the
identical input data. That is, the data 1 and data 2 input to the
modulators 11 and 21 at the time of adjusting the polarization
multiplexed optical transmitter have the same data pattern as each
other. However, for the modulator provided in the polarization
multiplexed optical transmitter according to the embodiment, as is
obvious from the drive voltage-output optical power characteristic
illustrated in FIG. 5 or FIG. 6, even when the input data is
reversed, the optical power of the output signal does not change.
Therefore, the data pattern of the data for generating the H
polarization signal may be the reversed pattern of the data for
generating the V polarization signal. That is, at the time of
adjusting the polarization multiplexed optical transmitter, the
data 1 input to the modulator 11 may be the reversed pattern of the
data 2 input to the modulator 21.
[0086] FIG. 7 illustrates the polarization components (H axis
component and V axis component) of the polarization multiplexed
optical signal generated at the time of adjustment. Here, the H
axis component propagates the output signal of the modulator
illustrated in FIG. 3A, and the V axis component propagates the
output signal of the modulator illustrated in FIG. 3B. The input
data for each of the modulators are the same as each other. In this
case, the H polarization signal has a local minimum of the optical
power when the input data change. In contrast, the V polarization
signal has the peak of the optical power when the input data
changes.
[0087] FIG. 8 illustrates the polarization components of the
polarization multiplexed optical signal at the time of data
transmission. At the time of data transmission, both the modulators
11 and 21 are controlled to the state illustrated in FIG. 3A. The
input data for the modulators 11 and 21 are independent from each
other.
[0088] FIG. 9 illustrates the optical intensity of the polarization
multiplexed optical signal and each polarization component
generated at the time of adjusting the polarization multiplexed
optical transmitter. In FIG. 9, the power of the polarization
multiplexed optical signal is normalized to "1", and the symbol
time period is normalized to "1".
[0089] The intensity (or the optical power) of the polarization
multiplexed optical signal is the sum of the optical intensity of
the H polarization component and the intensity of the V
polarization component. Here, the H polarization signal and the V
polarization signal are optical signals in reverse phases from each
other. Therefore, in the ideal state in which the following three
conditions are satisfied, the sum of the optical intensity of the H
polarization component and the intensity of the V polarization
component (that is, the optical intensity of the polarization
multiplexed optical signal) is constant. In the example illustrated
in FIG. 9, the optical intensity of the polarization multiplexed
optical signal is constantly "1". [0090] (1) the timing error
between the polarization is zero [0091] (2) the Tr/Tf difference
between the polarization is zero (the Tr/Tf of the H polarization
signal and the V polarization signal are the same) [0092] (3) the
optical power difference between the polarizations is zero (the
optical powers of the H polarization signal and the V polarization
signal are the same)
[0093] In other words, when one or more of the above three
conditions are not satisfied, the optical intensity of the
polarization multiplexed optical signal changes from "1". That is,
when one or more of the above three conditions are not satisfied,
the optical intensity waveform of the polarization multiplexed
optical signal is distorted in the time period in which the input
data changes. Therefore, the polarization multiplexed optical
transmitter according to the embodiment monitors the distortion of
the optical intensity of the polarization multiplexed optical
signal, and performs feedback control so as to compensate for the
distortion. As a result of the feedback control, the above three
conditions are satisfied, improving the balance between the
polarizations and improving the transmission characteristics of the
polarization multiplexed optical signal.
[0094] FIG. 10A-FIG. 10C illustrate the relationship between the
timing error between the polarizations and the optical intensity of
the polarization multiplexed optical signal. The optical intensity
(H+V) of the polarization multiplexed optical signal is the sum of
the optical intensity of the H polarization signal and the V
polarization signal.
[0095] FIG. 10A illustrates the state in which the H polarization
signal is delayed behind the V polarization signal. In this case,
the optical intensity of the polarization multiplexed optical
signal has a positive pulse in the period in which the input data
changes, and has a negative pulse after the positive pulse. Here,
the "positive pulse" represents the state in which the optical
intensity is larger than the steady-state level, and the "negative
pulse" represents the state in which the optical intensity is
smaller than the steady-state level. Here, the steady-state level
indicates the optical intensity in the time period in which the
input data is not changing, which is "1" in FIG. 10A-FIG. 10C. When
an optical intensity waveform in such a shape is detected, it is
determined that the H polarization signal is delayed behind the V
polarization signal.
[0096] When the H polarization signal is delayed behind the V
polarization signal, the signal controller 46 reduces the delay
time of the delay element 14 and/or increases the delay time of the
delay element 24, so as to make the optical intensity of the
polarization multiplexed optical signal constant (that is, to
reduce the amplitude of the pair of the positive pulse and the
negative pulse). Meanwhile, the timing error between the
polarizations maybe adjusted in other methods. For example, a
variable optical delay device may be provided on the output side of
at least one of the modulators 11 and 21, and the timing error
maybe adjusted by controlling the delay time of the variable
optical delay device. Alternatively, the timing error between the
polarizations may be adjusted by controlling the output timing of
the data generators 31 and 32. For example, in a case where the
data generators 31 and 32 are configured to have a D-flip flop
circuit, the delay elements 14 and 24 may be realized by
controlling the phase of a clock signal that instructs the output
timing of the D-flip flop circuit.
[0097] FIG. 10B illustrates a state in which the timing error is
larger than in FIG. 10A. As the timing error becomes larger, the
amplitude and/or the pulse width of the optical intensity waveform
of the polarization multiplexed optical signal becomes large. That
is, by monitoring the amplitude and/or the pulse width of the
optical intensity waveform of the polarization multiplexed optical
signal, the timing error between the polarizations maybe detected.
Therefore, the adjustment amount of the delay elements 14 and 24
may be calculated based on the amplitude or the pulse width of the
optical intensity waveform.
[0098] When the timing error between the polarizations is larger
than one symbol time period, the modulators 11 and 21 are driven by
data that are different from each other for each symbol. In this
case, the amplitude of the optical intensity waveform of the
polarization multiplexed optical signal takes the maximum value. In
other words, when the amplitude of the optical intensity waveform
of the polarization multiplexed optical signal is larger than a
specified threshold level, it is determined that the timing error
between the polarizations is larger than one symbol time
period.
[0099] FIG. 10C illustrates a state in which the V polarization
signal is delayed behind the H polarization signal. In this case,
the optical intensity of the polarization multiplexed optical
signal has a negative pulse in the time period in which the input
data changes, and has a positive pulse after the negative pulse.
That is, when the optical intensity waveform in such a shape is
detected, it is determined that the V polarization signal is
delayed behind the H polarization signal. Thus, by monitoring the
shape of the optical intensity waveform of the polarization
multiplexed optical signal, the adjustment direction of the delay
elements 14 and 24 is decided.
[0100] FIG. 11A-FIG. 11C illustrate the relationship between the
Tr/Tf difference between the polarizations and the optical
intensity. FIG. 11A illustrates a state in which the Tr/Tf of the V
polarization signal is larger than the Tr/Tf of the H polarization
signal. Specifically, in FIG. 11A, the rising time Tr of the V
polarization signal is longer than that of the H polarization
signal. In addition, the falling time Tf of the V polarization
signal is longer than that of the H polarization signal. In this
case, the optical intensity of the polarization multiplexed optical
signal has two positive pulses in the time period in which the
input data changes. That is, when an optical intensity waveform
having such a shape is detected, it is determined that the Tr/Tf of
the V polarization signal is larger than the Tr/Tf of the H
polarization signal.
[0101] When the Tr/Tf of the V polarization signal is larger than
the Tr/Tf of the H polarization signal, the signal controller 46
reduces the drive current of the driver 13 and/or increases the
drive current of the driver 23, so as to make the optical intensity
of the polarization multiplexed optical signal constant (that is,
to reduce the amplitude of the pair of positive pulses). Meanwhile,
the Tr/Tf of the polarization signal may be adjusted in other
methods. For example, the Tr/Tf may be adjusted by controlling the
transfer characteristic (such as the band of the low pass filter)
of the drivers 13 and 23.
[0102] FIG. 11B illustrates a state in which the Tr/Tf difference
is larger than in FIG. 11A. When the Tr/Tf difference between the
polarizations becomes larger, the amplitude and/or the pulse width
of the optical intensity waveform of the polarization multiplexed
optical signal becomes large. That is, by monitoring the amplitude
and/or the pulse width of the optical intensity waveform of the
polarization multiplexed optical signal, the Tr/Tf difference
between the polarizations maybe detected. Therefore, the adjustment
amount of the drivers 13 and 23 may be calculated based on the
amplitude and/or the pulse width of the optical intensity
waveform.
[0103] FIG. 11C illustrates a state in which the Tr/Tf of the H
polarization signal is larger than the Tr/Tf of the V polarization
signal. In this case, the optical intensity of the polarization
multiplexed optical signal has two negative pulses in the time
period in which the input data changes. That is, when an optical
intensity waveform having such a shape is detected, it is
determined that the Tr/Tf of the H polarization signal is larger
than the Tr/Tf of the V polarization signal. Thus, by monitoring
the shape of the optical intensity waveform of the polarization
multiplexed optical signal, the adjustment direction of the drivers
13 and 23 is decided.
[0104] FIG. 12A-FIG. 12C illustrate the relationship between the
optical power difference between the polarizations and the optical
intensity. FIG. 12A illustrates a state in which the optical power
of the V polarization signal is larger than the optical power of
the H polarization signal. In this case, the optical intensity of
the polarization multiplexed optical signal has one positive pulse
in the time period in which the input data changes. That is, when
an optical intensity waveform having such a shape is detected, it
is determined that the optical power of the V polarization signal
is larger than the optical power of the H polarization signal.
[0105] When the optical power of the V polarization signal is
larger than the optical power of the H polarization signal, the
signal controller 46 reduces the attenuation amount of the optical
attenuator 15 and/or increases the attenuation amount of the
optical attenuator 25, so as to make the optical intensity of the
polarization multiplexed optical signal constant (that is, to
reduce the amplitude of the pulse). Meanwhile, the optical power of
the polarization signal maybe adjusted in other methods. For
example, the optical power may be adjusted by controlling the
amplitude of the output signal of the drivers 13 and 23.
Alternatively, for example if the modulators 11 and 21 are
Mach-Zehnder LN modulator, the optical power may be adjusted by
controlling the bias of the LN modulator.
[0106] FIG. 12B illustrates a state in which the optical power
difference is larger than in FIG. 12A. When the optical power
difference between the polarizations becomes larger, the amplitude
of the optical intensity waveform of the polarization multiplexed
optical signal becomes large. That is, by monitoring the amplitude
the optical intensity waveform of the polarization multiplexed
optical signal, the optical power difference between the
polarizations may be detected. Therefore, the adjustment amount of
the optical attenuators 15 and 25 may be calculated based on the
amplitude of the optical intensity waveform.
[0107] FIG. 12C illustrates a state in which the optical power of
the H polarization signal is larger than the optical power of the V
polarization signal. In this case, the optical intensity of the
polarization multiplexed optical signal has one negative pulse in
the time period in which the input data changes. That is, when an
optical intensity waveform having such a shape is detected, it is
determined that the optical power of the H polarization signal is
larger than the optical power of the V polarization signal. Thus,
by monitoring the shape of the optical intensity waveform of the
polarization multiplexed optical signal, the adjustment direction
of the optical attenuators 15 and 25 is decided.
[0108] Each element (the delay element, the driver, the optical
attenuator etc.) may be adjusted in the dithering method. In the
dithering method, for example, when controlling the optical power
difference between polarizations, a low-frequency signal is
superimposed on the DC voltage controlling the attenuation amount
of the optical attenuators 15 and/or 25. Accordingly, the optical
intensity of the generated polarization multiplexed optical signal
contains a low-frequency signal component. Then, the controller 43
controls the DC voltage that controls the attenuation amount of the
optical attenuators 15 and/or 25 using the detected low-frequency
signal component.
[0109] Thus, the polarization multiplexed optical transmitter
according to the embodiment detects the timing error, the Tr/Tf
difference, and the optical power difference between polarizations
by analyzing the optical intensity waveform of the polarization
multiplexed optical signal. Then, the operation state of the
polarization multiplexed optical transmitter is adjusted by
feedback control so as to compensate for the detected timing error,
Tr/Tf difference, and optical power difference. Therefore, by
transmitting data after such adjustment, the transmission
characteristic of the polarization multiplexed optical signal
becomes good.
[0110] The three factors (the timing error, the Tr/Tf difference,
the optical power difference) described above that degrade the
characteristic of the polarization multiplexed optical signal occur
independently from each other. That is, the three factors may occur
at the same time. Hereinafter, the adjustment method in a case in
which the three factors occur at the same time is described.
[0111] The power of the H polarization signal (H.sub.power)
generated by one of the modulators 11 and 21 (the modulator 11
hereinafter) is expressed by the following function. This H
polarization signal corresponds to the output optical signal
illustrated in FIG. 5,
H Power = A .times. 1 + COS .theta. H 2 [ W ] ##EQU00001## .theta.
H = V DRV_H ( t ) V .pi. .times. .pi. [ rad ] ##EQU00001.2##
"A" represents the optical output peak power of the modulator 11.
V.sub.DRV.sub.--.sub.H(t) is a function of time and represents the
drive voltage of the Mach-Zehnder interferometer provided in the
modulator 11. V.pi. represents the drive voltage for shifting the
optical phase by .pi. in the Mach-Zehnder interferometer.
[0112] The power of the V polarization signal (V.sub.power)
generated by the other of the modulators 11 and 21 (the modulator
21 hereinafter) is expressed by the following function. This V
polarization signal corresponds to the output optical signal
illustrated in FIG. 6,
V Power = B .times. 1 + COS .theta. V 2 [ W ] ##EQU00002## .theta.
V = V DRV_V ( t ) V .pi. .times. .pi. + .pi. [ rad ]
##EQU00002.2##
"B" represents the optical output peak power of the modulator 21.
V.sub.DRV.sub.--.sub.V(t) is a function of time and represents the
drive voltage of the Mach-Zehnder interferometer provided in the
modulator 21.
[0113] The power of the polarization multiplexed optical power is
the sum of the powers of the H polarization signal and the V
polarization signal, which is represented by the following
expression.
Function representing the polarization multiplexed optical
signal=H.sub.Power+V.sub.Power
[0114] In the function representing the polarization multiplexed
optical signal, the timing error between the polarizations (or the
delay difference) is expressed by the time difference between
V.sub.DRV.sub.--.sub.H(t) and V.sub.DRV.sub.--.sub.V(t). The Tr/Tf
difference between the polarizations is expressed by the difference
between the differentiation of V.sub.DRV.sub.--.sub.H(t) with
respect to time and the differentiation of
V.sub.DRV.sub.--.sub.V(t) with respect to time. The differentiation
of V.sub.DRV.sub.--.sub.H(t) and V.sub.DRV.sub.--.sub.V(t) with
respect to time is proportional to the differentiation of
.theta..sub.H and .theta..sub.V with respect to time, respectively.
The optical power difference between the polarizations is expressed
by the difference between A and B.
[0115] Here, assuming that the timing error, the Tr/Tf difference
and the optical power difference between the polarizations are all
zero, V.sub.DRV.sub.--.sub.H(t)=V.sub.DRV.sub.--.sub.V(t) and A=B
is given for the function described above. Then the optical
intensity of the polarization multiplexed optical signal becomes
constantly "A". That is, no distortion is generated in the optical
intensity waveform of the polarization multiplexed optical
signal.
[0116] The method for adjusting the balance between the
polarization multiplexed optical signal that is degraded due to the
three factors described above is realized by the following
procedures, for example.
[0117] In procedure A1, the polarization multiplexed optical signal
in the time period in which the optical intensity modulation is
made is extracted, Then, with the time period as one cycle of
calculation, the extracted intensity modulated signal is expanded
to Fourier series. That is, the extracted signal is expressed by a
plurality of frequency spectra with respect to time.
[0118] In procedure A2, a fitting process is performed using the
following three variables so that the output function of the
optical intensity monitor 42 matches the function obtained in the
procedure A1. [0119] variable 1: V.sub.DRV.sub.--.sub.H(t) and
V.sub.DRV.sub.--.sub.V(t) [0120] variable 2: the differentiation of
V.sub.DRV.sub.--.sub.H(t) with respect to time and the
differentiation of V.sub.DRV.sub.--.sub.V(t) with respect to time
[0121] variable 3: A and B
[0122] In procedure A3, the following (1)-(3) are calculated based
on the processing result obtained in the procedure A2. [0123] (1)
the difference between V.sub.DRV.sub.--.sub.H(t) and
V.sub.DRV.sub.--.sub.V(t) [0124] (2) the difference between the
differentiation of V.sub.DRV.sub.--.sub.H(t) with respect to time
and the differentiation of V.sub.DRV.sub.--.sub.V(t) with respect
to time [0125] (3) the difference between A and B Then, these
calculation results correspond to the timing error, the difference
in the rising/falling times of the optical intensity modulation,
the optical power between the polarizations, respectively.
[0126] In procedure A4, based on the results obtained in the
procedure A3, corresponding elements (the delay elements 14 and 24,
the drivers 13 and 23, the optical attenuators 15 and 25, etc.) are
controlled so as to make each difference value (the timing error,
the difference in the rising/falling times of the optical intensity
modulation, the optical power difference) small.
[0127] In procedure A5, the procedures 1-3 are repeated for
specified times.
[0128] In procedure A6, whether or not the three difference values
are within a specified acceptable range is determined. When the
difference values are within the acceptable range, the adjustment
process is terminated. On the other hand, the difference values
exceed the acceptable range, the process returns to the procedure
A4.
[0129] The waveform distortion of the polarization multiplexed
optical signal is approximately expressed by the sum of a function
F1 representing the waveform distortion generated due to the timing
error, a function F2 representing the waveform distortion generated
due to the Tr/Tf difference, and a function F3 representing the
waveform distortion generated due to the optical power difference.
Here, if the characteristics of the Mach-Zehnder interferometer of
the modulators 11 and 21 have been detected or calculated, the
respective functions F1-F3 may be derived in advance. That is,
based on the characteristics of the Mach-Zehnder interferometer of
the modulators 11 and 21, the distortion component of the optical
intensity waveform of the polarization multiplexed optical signal
may be separated into functions F1, F2 and F3.
[0130] Alternatively, in the function (=H.sub.Power+V.sub.Power),
for example, when
"V.sub.DRV.sub.--.sub.H(t)=V.sub.DRV.sub.--.sub.V(t)" and "the
differentiation of V.sub.DRV.sub.--.sub.H(t) with respect to
time=the differentiation of V.sub.DRV.sub.--.sub.V(t) with respect
to time" are given, the function F3 representing the distortion of
the optical intensity waveform generated due to the optical power
difference between the polarizations is obtained. Then, by scanning
parameters A and B representing the amplitude of the polarization
signals in the function F3, a template of the function representing
the distortion due to the optical power difference between the
polarizations may be created. The same applies to the functions
representing the timing error and Tr/Tf difference between the
polarizations. Therefore, by comparing the optical intensity
waveform of the polarization multiplexed optical signal obtained by
the optical intensity monitor 42 and the template of each function,
the function component for the timing error, the Tr/Tf difference
and the optical time difference may be extracted.
[0131] In addition, while the Fourier series expansion and fitting
technique are used in the procedures described above, the
adjustment method according to the embodiment is not limited to
this. That is, other methods for comparing two functions and making
the difference small by adjusting a variable based on the
comparison result have been known, and the balance between the
polarizations may be adjusted using those methods.
[0132] Another method for adjusting the balance between the
polarizations of the polarization multiplexed optical signal
degraded due to the three factors described above is realized by
the following procedures, for example.
[0133] In procedure B1, the optical intensity waveform of the
polarization multiplexed optical signal is analyzed. Here, it is
assumed that the optical intensity waveform illustrated in FIG. 13A
has been obtained. That is, it is assumed that the H polarization
signal and the V polarization signal are in the following state.
[0134] (1) the H polarization signal is delayed behind the V
polarization signal [0135] (2) the Tr/Tf of the H polarization
signal is larger than the Tr/Tf of the V polarization signal [0136]
(3) the optical power A of the H polarization signal is smaller
than the optical power B of the V polarization signal (A/B=0.8)
[0137] In procedure B2, the delay amount of the delay elements 14
and 24 is adjusted so that the optical intensity waveform of the
polarization multiplexed optical signal becomes a symmetric on the
time axis. Here, the optical intensity waveform of the polarization
multiplexed optical signal becomes symmetric on the time axis when
the timing error between the H polarization signal and the V
polarization signal is zero. Therefore, the controller 43 adjusts
the timing error between the H polarization signal and the V
polarization signal while monitoring the optical intensity waveform
of the polarization multiplexed optical signal. In the optical
intensity waveform illustrated in FIG. 13A, a positive pulse
appears first, and a negative pulse appears after the positive
pulse. Therefore, it is determined that the H polarization signal
is delayed behind the V polarization signal. Accordingly, control
to reduce the delay amount of the delay element 14 and/or to
increase the delay amount of the delay element 24 is performed. As
a result, the optical intensity waveform illustrated in FIG. 13B is
obtained.
[0138] In procedure B3, the attenuation amount of the optical
attenuators 15 and 25 is adjusted so that, for the optical
intensity waveform illustrated in FIG. 13B, the optical intensity
in the center area (that is, TIME=0.5) and the optical intensity in
the steady state match each other. Here, in the optical intensity
waveform of the polarization multiplexed optical signal, the
optical intensity in the center area and the optical intensity in
the steady state match each other when the power difference between
the H polarization signal and the V polarization signal is zero.
Therefore, the controller 43 adjusts the optical power of the H
polarization signal and/or the V polarization signal while
monitoring the optical intensity waveform of the polarization
multiplexed optical signal. In this example, the optical intensity
in the center area is "1", and the optical intensity in the steady
state is "0.8". Therefore, in this case, it is determined that the
optical power of the H polarization signal is smaller than the
optical power of the V polarization signal. In this case, control
to reduce the attenuation amount of the optical attenuator 15
and/or to increase the attenuation amount of the optical attenuator
25 is performed. Here, the optical attenuator 15 is controlled so
that the optical intensity in the steady state becomes "1". As a
result, the optical intensity waveform illustrated in FIG. 14A is
obtained.
[0139] In procedure B4, the drive voltage of the drivers 13 and/or
23 is controlled so that the optical intensity of the polarization
multiplexed optical signal becomes constant. In the optical
intensity waveform illustrated in FIG. 14A, two negative pulses
have appeared. In this case, it is determined that the Tr/Tf of the
H polarization signal is larger than the Tr/Tf of the V
polarization signal. In this case, control to increase the drive
current of the driver 13 and/or to reduce the drive current of the
driver 23 is performed. As a result, as illustrated in FIG. 14B,
the optical intensity of the polarization multiplexed optical
signal becomes constant.
[0140] The procedures B1-B4 are automatically performed by the
controller 43 for example. In addition, the procedures B1-B4 may be
performed with intervention by a human. In this case, for example,
the optical intensity waveform of the polarization multiplexed
optical signal is displayed on an oscilloscope. Then, by referring
to the optical intensity waveform displayed on the oscilloscope,
each element (the delay element, the driver, the optical
attenuator) is adjusted by manual operation.
[0141] The optical intensity waveform may be easily monitored by
using a high-speed oscilloscope (or a monitor device using a
lowpass filter) as the optical intensity monitor 42. In addition,
since the optical intensity monitor will suffice as long as it
detects the distortion of the optical intensity waveform, it may be
a device that detects the frequency band of the waveform
distortion. That is, for example, it maybe an amplifier having a
band bass filter that passes the frequency band of the waveform
distortion through, or may be a simple spectrum analyzer.
[0142] The order to perform the procedures B1-B4 is not limited to
the example described above. In addition, only a part of the
procedures B1-B4 may be performed.
[0143] Furthermore, the modulation scheme of the modulators 11 and
21 is not limited to BPSK, and may be another MPSK (M=2.sup.n) such
as QPSK. When the number of bits per one symbol increases, control
to optimize the balance between the polarization becomes
complicated, but the procedures are similar to those in the case
for BPSK described above.
[0144] Next, the variation of the polarization multiplexed optical
transmitter according to the embodiment is described.
[0145] <Variations of the BPSK Modulator>
[0146] Each modulator is not limited to the single drive
configuration illustrated in FIG. 3A and FIG. 3B, and may be the
dual drive configuration illustrated in FIG. 15A and FIG. 15B. In
this case, at the time of data transmission, the modulators 11 and
21 are controlled to the phase status illustrated in FIG. 15A. In
the adjustment mode, one of the modulators 11 and 21 is controlled
to the phase status illustrated in FIG. 15A, and the other of the
modulators 11 and 21 is controlled to the phase status illustrated
in FIG. 15B.
[0147] The optical paths P1 and P2 of each modulator may be coupled
by an X coupler as illustrated in FIG. 16A and FIG. 16B. In this
case, at the time of data transmission, the modulators 11 and 21
are controlled to the phase status illustrated in FIG. 16A. That
is, phase .pi./2 is given to the optical path P1. The phase .pi./2
includes .pi./2+2n.pi. or -.pi./2+2n.pi. (n is an integer other
than zero). Then, the modulated optical signal is output from a
straight port for the optical path P1 (that is, a cross port for
the optical path P2). In contrast, in the adjustment mode, one of
the modulators 11 and 21 is controlled to the phase status
illustrated in FIG. 16A, and the other of the modulators 11 and 21
is controlled to the phase status illustrated in FIG. 16B. The
status illustrated in FIG. 16B is realized by giving phase .pi./2
to the optical path P2.
[0148] <QPSK>
[0149] While the modulators 11 and 21 are BPSK modulators in the
description above, the polarization multiplexed optical transmitter
according to the embodiment is not limited to this configuration.
That is, the modulators 11 and 21 may be configured to perform
modulation in another scheme.
[0150] FIG. 17A illustrates the configuration of a modulator that
performs QPSK modulation. In this modulator, the input CW light is
spilt equally and guided to the optical paths P1 and P2. A
Mach-Zehnder interferometer is provided on each of the optical
paths P1 and P2. In this example, each Mach-Zehnder interferometer
is the same as the BPSK modulator illustrated in FIG. 3A.
Meanwhile, each Mach-Zehnder interferometer is not limited to this
configuration, and the configuration illustrated in FIG. 15A or
FIG. 16A may be adopted.
[0151] In QPSK, 2-bit data (I arm data and Q arm data) is input
with every symbol. The Mach-Zehnder interferometer on the optical
path P1 is driven by the I arm data, and the Mach-Zehnder
interferometer on the optical path P2 is driven by the Q arm data.
In addition, in QPSK, phase difference .pi./2 is given between the
optical paths P1 and P2. The phase .pi./2 includes .pi./2+2n.pi. or
-.pi./2+2n.pi. (n is an integer other than zero).
[0152] FIG. 18A and FIG. 18B illustrate the operation of the
modulator illustrated in FIG. 17A and FIG. 17B. In the QPSK
modulator illustrated in FIG. 17A, when the I arm data is "0", the
electric field vector indicated by a signal point al in FIG. 18A is
obtained. When the I arm data is "1", the electric field vector
indicated by a signal point a2 is obtained. Similarly, when the Q
arm data is "0", "1", the electric field vector indicated by signal
points a3, a4 is obtained, respectively. Therefore, when the input
2-bit data is "00", "01", "11", "10", the electric field vector of
the optical signal output from the QPSK modulator is indicated by
the signal points b1, b2, b3, b4, respectively.
[0153] The electric field vector of the optical signal output from
the QPSK modulator transits when the input 2-bit data changes. For
example, when the 2-bit data changes from "00" to "11", the
electric field vector transits from the signal point b1 to the
signal point b3 through the origin. Here, the power of the optical
signal is proportional to the square of the distance from the
origin to a corresponding position. Therefore, during this
transition of 2-bit data, the power of the optical signal once
becomes zero. Meanwhile, when the 2-bit data changes from "00" to
"01", the electric field vector transits from the signal point b1
to the signal point b2. In this transition process, the power of
the optical signal once decreases, but does not become zero.
[0154] As described above, when the first bit and the second bit of
the 2-bit data both change, the power of the optical signal once
becomes zero during the transition. When only one of the bits of
the 2-bit data changes, the power of the optical signal once
decreases but does not become zero during the transition.
Meanwhile, when the 2-bit data does not change, the power of the
optical signal does not change.
[0155] In the adjustment mode, one of the modulators 11 and 21 is
controlled to the state illustrated in FIG. 17B. In this example,
the Mach-Zehnder interferometer on the optical paths P1 and P2 is
respectively controlled to the state illustrated in FIG. 3B. That
is, the phase difference between the optical paths of each
Mach-Zehnder interferometer is shifted by .pi. from the target
value. In this case, in each Mach-Zehnder interferometer, a pair of
optical signals output from the pair of optical paths cancels each
other. For this reason, the output of the QPSK modulator enters the
no-light-emission state as illustrated in FIG. 18B. That is, the
electric field vector of the output light is represented by the
origin, as illustrated in FIG. 18B.
[0156] However, when both of the first bit and the second bit of
the 2-bit data change, the state of the optical signal transits
from the origin to the signal point c1 (one of the signal points
b1-b4 in FIG. 18A) and then returns to the origin. In addition,
when only one of the bits of the 2-bit data changes, the state of
the optical signal transits from the origin to the signal point c2
(one of the signal points a1-a4 in FIG. 18A) and then returns to
the origin.
[0157] FIG. 19 illustrates the polarization component of the
polarization multiplexed optical signal in which two QPSK modulated
optical signals are multiplexed. In the adjustment mode, the H
polarization propagates the output signal of the modulator
illustrated in FIG. 17A, and the V polarization propagates the
output signal of the modulator illustrated in FIG. 17B. The 2-bit
data (control data 1 and 2) input to the modulators (the modulators
11 and 12) are the same each other. In this case, the H
polarization signal has a local minimum value of the optical power
when the input data changes. At this time, the local minimum value
at the time when both of the bits in the 2-bit data change becomes
smaller than the local minimum value at the time when only one of
the bits of the 2-bit data changes. Meanwhile, the V polarization
signal has the peak of the optical power when the input data
changes. At this time, the peak at the time when both of the bits
in the 2-bit data change becomes larger than the peak at the time
when only one of the bits of the 2-bit data changes.
[0158] FIG. 20 illustrates the power of the polarization
multiplexed optical signal in which QPSK modulated optical signals
are multiplexed and each polarization component. In FIG. 20, the
power of the polarization multiplexed optical signal is normalized
to "1", and symbol time period is normalized to "1".
[0159] The power of the polarization multiplexed optical signal is
the sum of the powers of the H polarization and the V polarization.
Here, the H polarization signal and the V polarization signal are
optical signals in reverse phases from each other. Therefore, in an
ideal state in which the three conditions described above (the
timing error is zero, the Tr/Tf difference is zero, the optical
power difference is zero between the polarizations) are satisfied,
the sum of the powers of the H polarization signal and the V
polarization signal (that is, the power of the polarization
multiplexed optical signal) is constant.
[0160] Note that, as illustrated in FIG. 20, when both of the bits
of the 2-bit data change, the optical powers of the H polarization
signal and the V polarization signal change as represented by
waveform H2 and waveform V2, respectively. When only one of the
bits of the 2-bit data changes, the optical powers of the H
polarization signal and the V polarization signal change as
represented by waveform H1 and waveform V1, respectively. In either
case, when the three conditions described above are satisfied, the
power of the polarization multiplexed optical signal is
constant.
[0161] FIG. 21A-FIG. 21C illustrate the relationship between the
timing error between the polarizations and the optical intensity of
the polarization multiplexed optical signal in an optical
transmitter using QPSK. FIG. 21A illustrates a state in which the H
polarization signal is delayed behind the V polarization. FIG. 21B
illustrates a state in which the timing error is larger than in
FIG. 21A. FIG. 21C illustrates a state in which the V polarization
signal is delayed behind the H polarization signal.
[0162] H1 and V1 respectively represent the optical intensity of
the H polarization signal and the V polarization signal at the time
when only one of the bits of corresponding 2-bit data transmitted
in QPSK changes. In this case, the optical intensity of the
polarization multiplexed optical signal is represented by H1+V1.
Meanwhile, H2 and V2 respectively represent the optical intensity
of the H polarization signal and the V polarization signal at the
time when both of the bits of corresponding 2-bit data transmitted
in QPSK changes. In this case, the optical intensity of the
polarization multiplexed optical signal is represented by
H2+V2.
[0163] FIG. 22A-FIG. 22C illustrate the relationship between the
Tr/Tf difference between the polarizations and the optical
intensity of the polarization multiplexed optical signal in an
optical transmitter using QPSK. FIG. 22A illustrates a state in
which the Tr/Tf of the V polarization signal is larger than the
Tr/Tf of the H polarization signal. FIG. 22B illustrates a state in
which the Tr/Tf difference is larger than in FIG. 22A. FIG. 22C
illustrates a state in which the Tr/Tf of the H polarization signal
is larger than the Tr/Tf of the V polarization signal.
[0164] FIG. 23A-FIG. 23C illustrate the relationship between the
optical power difference between the polarizations and the optical
intensity of the polarization multiplexed optical signal in an
optical transmitter using QPSK. FIG. 23A illustrates a state in
which the optical power of the V polarization signal is larger than
the optical power of the H polarization signal. FIG. 23B
illustrates a state in which the optical power difference is larger
than in FIG. 23A. FIG. 23C illustrates a state in which the optical
power of the H polarization signal is larger than the optical power
of the V polarization signal.
[0165] As described above, when the modulation scheme of the
modulators 11 and 21 is QPSK, the optical intensity of the
polarization multiplexed optical signal represents the same trend
as in the case in which the modulation scheme of the modulators 11
and 21 is BPSK. Therefore, when the modulation scheme of the
modulators 11 and 21 is QPSK, the balance between the polarizations
may also be adjusted by the method described above. That is, the
method described above may be applied to MPSK (M=2.sup.n)
modulation.
[0166] <Configuration with Two Light Sources>
[0167] In the configuration illustrated in FIG. 2, the CW light
generated by the light source 1 is split and guided to the
modulators 11 and 21. However, the present invention is not limited
to this configuration. That is, the polarization multiplexed
optical transmitter may be configured to have two light sources 4
and 5, as illustrated in FIG. 24. In this case, the CW lights
generated by the light sources 4 and 5 are guided to the modulators
11 and 21, respectively. At this time, the wavelengths of the CW
lights generated by the light sources 4 and 5 may be the same as
each other or may be different from each other. However, when the
wavelengths are different from each other, it is preferable that
these wavelengths are selected within a wavelength range in which
the wavelength dependence of the photo detector of the optical
intensity monitor 42 is small enough to be regarded as
negligible.
[0168] Meanwhile, in the configuration in which a light source is
provided respectively for each modulator, the output power of the
light sources may be controlled while the optical power difference
is adjusted. That is, for example, when the optical power of the H
polarization signal is higher than that of the V polarization
signal, the controller 43 may perform control to reduce the output
power of the light source 4 and/or to increase the output power of
the light source 5.
[0169] Next, the method for adjusting the polarization multiplexed
optical transmitter is described. FIG. 25 is a flowchart
illustrating the method for adjusting the polarization multiplexed
optical transmitter. The process in the flowchart is performed by
the controller 43. In addition, the process in this flowchart is
performed, for example, before the polarization multiplexed optical
transmitter starts data transmission, or at maintenance times.
[0170] In step S1, initial adjustment is performed. In the initial
adjustment, for example, the output power of the light source 1
(or, the light sources 4 and 5), the DC bias of the modulators 11
and 21, the attenuation amount of the optical attenuators 15 and 25
are respectively set to a target value. In addition, in a case in
which the modulators 11 and 21 performs QPSK (including DQPSK)
modulation, phase difference .pi./2 is given between the optical
paths P1 and P2 illustrated in FIG. 17A and FIG. 17B.
[0171] In step S2, the controller 43 makes the data generators 31
and 32 generate control data (data 1 and data 2) that satisfy the
following condition. [0172] "data 1=data 2" or "data 1=reversed
data 2" That is, the same data are input to the modulators 11 and
21. Alternatively, reverse data to each other may be input to the
modulators 11 and 21. Note that "Data 2_bar" in FIG. 25 indicates
reversed data of data 2.
[0173] In step S3, in one of the modulators 11 and 21, the phase
difference between the optical paths of the Mach-Zehnder
interferometer is shifted by .pi. from the target value. Here, in
the case in which the modulators 11 and 21 perform BPSK modulation,
for example, the modulator 11 is controlled to the state
illustrated in FIG. 3A, and the modulator 21 is controlled to the
state illustrated in FIG. 3B. Meanwhile, in the case in which the
modulators 11 and 21 perform QPSK modulation, for example, the
modulator 11 is controlled to the state illustrated in FIG. 17A,
and the modulator 21 is controlled to the state illustrated in FIG.
17B.
[0174] In step S4, the optical intensity waveform of the
polarization multiplexed optical signal is monitored, and feedback
control to match the characteristics of the H polarization signal
and the V polarization signal is performed. When minimizing the
timing error between the polarization signals, for example, the
delay time of the delay elements 14 and/or 24 is controlled by the
delay adjustment signal. When minimizing the Tr/Tf difference
between the polarizations, for example, the drive current of the
drivers 13 and/or 23 is adjusted by a drive adjustment signal. When
minimizing the optical power difference between the polarizations,
for example, the attenuation amount of the optical attenuators 15
and/or 25 is controlled by an optical power adjustment signal.
[0175] In step S5, the controller 43 makes the data generators 31
and 32 generate transmission data. In addition, the controller 43
controls the phase difference between the optical paths of the
Mach-Zehnder interferometer provided in the modulators 11 and 21 to
the target value. Data transmission starts after that.
[0176] FIG. 26 is a flowchart illustrating the method for adjusting
the balance between the polarizations of the polarization
multiplexed optical signal. The process in the flowchart
corresponds to S4 in FIG. 25.
[0177] In step S11, the controller 43 monitors the optical
intensity waveform of the polarization multiplexed optical signal.
The signal representing the optical intensity of the polarization
multiplexed optical signal is obtained by the optical intensity
monitor 42. In step S12, functions for the three distortion factors
described above (the timing error, the Tr/Tf difference, the
optical power difference) are extracted from the distortion
component of the optical intensity waveform of the polarization
multiplexed optical signal. The method for extracting functions for
the respective distortion factors is as described above. Note that
"function" includes pattern of pulse(s) of the optical intensity
waveform. The "pattern of pulse (s)" includes the number of pulses,
amplitude and/or width of the pulse, sign (positive or negative) of
the pulse, occurrence order of the pulses, and so on.
[0178] In steps S13-S15, the timing error between polarizations is
minimized. That is, in Step S13, the timing error between the H
polarization signal and the V polarization signal (or, the delay
time difference between the polarizations) is calculated based on
the function for the timing error extracted in step S12. In step
S14, the delay compensation amount corresponding to the timing
error is calculated. Here, for example, in the case in which the
delay time of the delay elements 14 and 24 is controlled by the
voltage, as the delay compensation amount, the voltage
corresponding to the timing error is calculated. In this case, it
is assumed that the correspondence relationship between the timing
error and the voltage has been obtained in advance. Then, in step
S15, the delay time of the delay elements 14 and/or 24 is adjusted
according to the calculated delay compensation amount.
[0179] In steps S16-S18, the Tr/Tf difference between the
polarizations is minimized. That is, in step S16, the Tr/Tf
difference between the polarizations is calculated based on the
function for the Tr/Tf difference extracted in step S12. In step
S17, the Tr/Tf compensation amount corresponding to the Tr/Tf
difference is calculated. For example, in the case in which the
response time of the drivers 13 and 23 is controlled by the drive
current, as the Tr/Tf compensation amount, the drive current
corresponding to the Tr/Tf difference is calculated. In this case,
it is assumed that the correspondence relationship between the
Tr/Tf difference and the drive current has been obtained in
advance. Then, in step S18, the drive current of the drivers 13
and/or 23 is adjusted according to the calculated Tr/Tf
compensation amount.
[0180] In steps S19-S21, the optical power difference between the
polarizations is minimized. That is, in step S19, the optical power
difference between the polarizations is calculated based on the
function for the optical power difference extracted in step S12. In
step S20, the optical power compensation amount corresponding to
the optical power difference is calculated. For example, in the
case in which the attenuation amount of the optical attenuators 15
and 25 is controlled by the applied voltage, as the optical power
compensation amount, the applied voltage corresponding to the
optical power difference is calculated. In this case, it is assumed
that the correspondence relationship between the optical power
difference and the applied voltage has been obtained in advance.
Then, in step S21, the attenuation amount of the optical
attenuators 15 and/or is adjusted according to the calculated
optical power compensation amount.
[0181] While the timing error, the Tr/Tf difference, and the
optical power difference are calculated and the corresponding
elements (the delay element, the driver, the optical attenuator)
are adjusted based on them in the flowchart illustrated in FIG. 26,
the control method according to the embodiment is not limited to
this procedure. That is, the compensation amount for controlling
each element does not need to be calculated directly. For example,
the polarization (the V polarization signal or the H polarization
signal) having higher optical power may be detected from the
distortion component of the optical intensity waveform of the
polarization multiplexed optical signal, and the attenuation amount
of the corresponding optical attenuator may be adjusted by a
specified adjustment amount according to the detection result. In
this case, so as to make the optical power difference between
polarizations smaller than a specified threshold, an operation to
adjust the attenuation amount is performed repeatedly by feedback
control. A similar method may be applied to the timing error and
the Tr/Tf difference.
[0182] Next, a polarization multiplexed optical transmitter and a
method for controlling the polarization multiplexed optical signal
according to another embodiment are described.
[0183] In the embodiment described above (the polarization
multiplexed optical transmitter illustrated in FIG. 2 or FIG. 24),
the adjustment of the balance between the polarizations of the
polarization multiplexed optical signal is performed during the
period in which data are not transmitted to a receiving station. In
contrast, in the embodiment described below, the balance between
the polarizations of the polarization multiplexed optical signal
may be controlled during data transmission as well.
[0184] FIG. 27 illustrates the configuration of the polarization
multiplexed optical transmitter according to another embodiment.
The polarization multiplexed optical transmitter is similar to the
configuration illustrated in FIG. 2. However, one of the pair of
modulators generates an optical signal for adjustment in addition
to the normal optical signal.
[0185] A modulator 51 generates a pair of optical signals according
to data 1 generated by the data generator 31. One of the optical
signals is a modulated optical signal H, which is guided to the
polarization beam combiner 3 through the optical attenuator 15. The
other of the optical signals is an adjustment optical signal, which
is a reversed phase signal of the modulated optical signal H. This
adjustment optical signal is guided to the polarization beam
combiner 53 through the optical attenuator 52.
[0186] The modulator 21 generates an optical signal according to
data 2 generated by the data generator 32 in the same manner as in
the configuration illustrated in FIG. 2. This optical signal is a
modulated optical signal V, which is guided to the beam splitter 54
through the optical attenuator 25. The beam splitter 54 splits the
modulated optical signal V and guides it to the polarization beam
combiners 3 and 53.
[0187] The polarization beam combiner 3 generates a polarization
multiplexed optical signal that carries the data 1 and the data 2
by combining the modulated optical signal H generated by the
modulator 51 and the modulated optical signal V generated by the
modulator 21. This polarization multiplexed optical signal is
transmitted to a receiving station through an optical fiber
transmission path. The polarization beam combiner 53 generates an
adjustment polarization multiplexed optical signal by combining the
adjustment optical signal generated by the modulator 51 and the
modulated optical signal V generated by the modulator 21. The
adjustment polarization multiplexed optical signal is guided to an
optical intensity monitor 55. The optical intensity monitor 55
detects the optical intensity of the adjustment polarization
multiplexed optical signal.
[0188] The controller 43 controls each element (the delay elements
14 and 24, the drivers 13 and 23, the optical attenuators 15, 25
and 52) based on the optical intensity and the waveform of the
adjustment polarization multiplexed optical signal detected by the
optical intensity monitor 55.
[0189] When the modulation scheme is BPSK, the modulator 51 is
realized by the configuration illustrated in FIG. 28A for example.
In the modulator illustrated in FIG. 28A, the input CW light is
split and guided to the optical paths P1 and P2. Phase difference
.pi./2 is given between the optical paths P1 and P2. In this
example, phase adjustment is made so that the optical path lengths
of the optical path P1 is longer than the optical path length of
the optical path P2 by the amount of .pi./2. In addition, the
optical paths P1 and P2 are coupled by an X coupler.
[0190] In the modulator configured as described above, a pair of
optical signals output through a first output port and a second
output port of the X coupler are complementary with each other,
Therefore, when the modulated optical signal H is output through
the first output port of the X coupler, an optical signal with the
reversed phase of the modulated optical signal H is output through
the second output port of the X coupler. Here, the optical signal
output through the second output port of the X coupler is used as
the adjustment optical signal illustrated in FIG. 27. That is, the
adjustment optical signal is an optical signal with the reversed
phase of the modulated optical signal H. Meanwhile, the optical
paths P1 and P2 do not have to be coupled by the X coupler. The X
coupler may be replaced by another optical coupling element to
output a pair of optical signals that are complementary with each
other.
[0191] While the modulator illustrated in FIG. 28A is a dual drive
configuration, the modulator 51 may be configured to be a single
drive configuration as illustrated in FIG. 28B. Also in this
configuration, the modulated optical signal H is output from the
first output port of the X coupler, and the adjustment optical
signal that is a reversed phase signal of the modulated optical
signal H is output from the second output port. In the
configuration illustrated in FIG. 28B, the input terminal of the
Mach-Zehnder interferometer is also configured with an X coupler.
In this configuration, the phase difference .pi./2 is realized by
guiding an optical signal input from one of the ports of the X
coupler to the pair of optical paths of the Mach-Zehnder
interferometer.
[0192] As described above, the modulator 51 generates the modulated
optical signal H and the adjustment optical signal. The modulator
21 generates the modulated optical signal V. Here, when the data 1
and 2 that drive the modulators 51 and 21 are the same with each
other, the modulated optical signals H and V are the same with each
other. In addition, the adjustment optical signal is a reversed
phase signal of the modulated optical signal V. The optical
intensity of the adjustment optical signal is the same as that of
the modulated optical signals H. Therefore, the modulated optical
signal V and the adjustment optical signal correspond to the
modulated optical signal H and the modulated optical signal V
generated at the time of adjustment in the configuration
illustrated in FIG. 2. Accordingly, in the polarization multiplexed
optical transmitter illustrated in FIG. 27, the balance between the
polarizations may be optimized in the same manner as in the
configuration in FIG. 2, by performing feedback control using the
adjustment polarization multiplexed optical signal in which the
adjustment optical signal output from the modulator 51 and the
modulated optical signal V output from the modulator 21 are
multiplexed.
[0193] FIG. 29 illustrates the configuration of data input to the
modulators in the polarization multiplexed optical transmitter
illustrated in FIG. 27. The data 1 and 2 respectively input to the
modulators 51 and 21 are in synchronization with each other. The
data 1 and 2 are stored in a frame of a specified length in this
example. Each frame has a header H and a body portion. The body
portion of each frame for the data 1 stores transmission data and
adjustment data d1, and the body portion of each frame for the data
2 stores transmission data and adjustment data d2. The adjustment
data d1 and d2 are stored in the same position that has been
determined in advance. The data pattern of the adjustment data d1
is the same as the adjustment data d2, or the reversed pattern of
the data d2. For example, if the data d1 is "010011", the data d2
is "010011" or "101100".
[0194] The adjustment data 1 and 2 are in synchronization with each
other as described above. Therefore, the adjustment data d1 and d2
are input to the modulators 51 and 21 regularly and at the same
time.
[0195] The controller 43 recognizes the timing at which the
adjustment data d1 and d2 are generated. Then, the controller 43
monitors the optical intensity of the adjustment polarization
multiplexed optical signal output through the polarization beam
combiner 53 in the time period in which the adjustment data 1 and 2
are generated. Further, the controller 43 controls the balance
between the polarizations of the polarization multiplexed optical
signal using the monitoring result. Note that the method for
adjusting the delay elements, the drivers and the optical
attenuators, etc. for controlling the balance between the
polarizations is the same as in the embodiments described
above.
[0196] The polarization multiplexed optical signal output through
the polarization beam combiner 53 s transmitted to a receiving
station through an optical fiber transmission path. The receiving
station recovers the data 1 and 2 by separating and demodulating
the polarization multiplexed optical signal. At this time, the
receiving station may remove and discard the adjustment data d1 and
d2.
[0197] As described above, in the embodiment illustrated in FIG.
27, the balance between the polarizations of the polarization
multiplexed optical signal is adjusted while transmitting data to
the receiving station. That is, the polarization multiplexed
optical transmitter is capable of dynamically controlling the
transmission characteristic of the polarization multiplexed optical
signal without stopping the data transmission service.
[0198] In the embodiment illustrated in FIG. 27, the modulation
scheme of the modulators 51 and 21 is not limited to BPSK, and may
be another modulation scheme. That is, the modulators 51 and 21 may
be, for example, another MPSK (M=2.sup.n) such as QPSK.
[0199] FIG. 30 is an example of the modulator 51 that performs QPSK
modulation. The modulator 51 that performs QPSK modulation has
first and second Mach-Zehnder interferometers, and CW light is
input to each Mach-Zehnder interferometer. In addition, I arm data
and Q arm data are given to the first and second Mach-Zehnder
interferometers, respectively. Each Mach-Zehnder interferometer is
a single drive configuration in FIG. 30, however, it may be
configured to be a dual drive configuration. In addition, each
output port of the second Mach-Zehnder interferometer has a phase
shift element for shifting the phase of the optical signal by
.pi./2.
[0200] In the modulator configured as described above, an optical
signal output from a second output port of the first Mach-Zehnder
interferometer and an optical signal output from a first port of
the second Mach-Zehnder interferometer are combined and output as a
modulated optical signal H that carries transmission data. On the
other hand, an optical signal output from a first output port of
the first Mach-Zehnder interferometer and an optical signal output
from a second output port of the second Mach-Zehnder interferometer
are combined and output as the adjustment optical signal.
Meanwhile, it is assumed that the adjustment data for (d1 and d2)
illustrated in FIG. 29 are inserted into the I arm data and Q arm
data at specified intervals, respectively.
[0201] Meanwhile, in the embodiment illustrated in FIG. 27, the
optical circuit for the adjustment polarization multiplexed optical
signal is different from the optical circuit for the main
polarization multiplexed optical signal transmitted to the
receiving station. For example, a portion of the modulated optical
signal V output from the modulator 21 is split by the beam splitter
54. For this reason, the characteristics (for example, the optical
path length, the bias value of the modulator/the characteristic of
the optical phase operation amount, the optical power, etc.) of the
main polarization multiplexed optical signal and the adjustment
polarization multiplexed optical signal may not match. Therefore,
in the configuration illustrated in FIG. 27, it is preferable that
a function to compensate for the difference in these
characteristics is provided.
[0202] As described above, according to the embodiments illustrated
in FIG. 2-FIG. 30, the balance between the polarizations of the
polarization multiplexed optical signal is improved or optimized.
Therefore, the transmission characteristics of the polarization
multiplexed optical signal are improved. As a result, the error
rate of the data recovered at the receiving station decreases, and
the reception quality improves. Furthermore, when data are
recovered by a coherent receiver and a digital signal processor in
the receiving station, the load of the digital signal processing is
reduced.
[0203] Note that there is no particular limitation on the
configuration or method of the receiver that receives the
polarization multiplexed optical signal generated by the
polarization multiplexed optical transmitter according to the
embodiments. For example, digital coherent reception may be used,
or direct reception using an interferometer maybe used. In
addition, the phase .pi./2 includes .pi./2+2n.pi. or -.pi./2+2n.pi.
(n is an integer).
[0204] All examples and conditional language recited herein are
intended for pedagogical purposes to aid the reader in
understanding the invention and the concepts contributed by the
inventor to furthering the art, and are to be construed as being
without limitation to such specifically recited examples and
conditions, nor does the organization of such examples in the
specification relate to a showing of the superiority and
inferiority of the invention. Although the embodiment (s) of the
present inventions has (have) been described in detail, it should
be understood that the various changes, substitutions, and
alterations could be made hereto without departing from the spirit
and scope of the invention.
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